@function px2em($px, $base-font-size: 75px) {
    @if (unitless($px)) {
        @warn "Assuming #{$px} to be in pixels, attempting to convert it into pixels for you";
        @return px2em($px + 0px); // That may fail.
    } @else if (unit($px) == em) {
        @return $px;
    }
    @return ($px / $base-font-size) * 1rem;
}
div{
    width: px2em(100px);
}